Geocoder¶ ↑

Geocoder adds object geocoding and database-agnostic distance calculations to Ruby on Rails. It’s as simple as calling fetch_coordinates! on your objects, and then using a named scope like Venue.near("Billings, MT").

Geocoder does not rely on proprietary database functions so finding geocoded objects in a given area is easily done using out-of-the-box MySQL or even SQLite.

Geocoder is currently compatible with Rails 2.x and Rails 3.

1. Install¶ ↑

Install either as a plugin:

script/plugin install git://github.com/alexreisner/geocoder.git

or as a gem:

# add to config/environment.rb:
config.gem "rails-geocoder", :lib => "geocoder", :source => "http://gemcutter.org/"

# at command prompt:
sudo rake gems:install

2. Configure¶ ↑

A) Get a Google Maps API key (see code.google.com/apis/maps/signup.html) and store it in a constant:

# eg, in config/initializers/google_maps.rb
GOOGLE_MAPS_API_KEY = "..."

B) Add latitude and longitude columns to your model:

script/generate migration AddLatitudeAndLongitudeToYourModel latitude:float longitude:float
rake db:migrate

C) Tell geocoder where your model stores its address:

geocoded_by :address

D) Optionally, auto-fetch coordinates every time your model is saved:

after_validation :fetch_coordinates

Note that you are not stuck with the latitude and longitude column names, or the address method. See “More On Configuration” below for details.

3. Use¶ ↑

Assuming obj is an instance of a geocoded class, you can get its coordinates:

obj.fetch_coordinates              # fetches and assigns coordinates
obj.fetch_coordinates!             # also saves lat, lon attributes

If you have a lot of objects you can use this Rake task to geocode them all:

rake geocode:all CLASS=YourModel

Once obj is geocoded you can do things like this:

obj.nearbys(30)                    # other objects within 30 miles
obj.distance_to(40.714, -100.234)  # distance to arbitrary point

To find objects by location, use the following named scopes:

Venue.near('Omaha, NE, US', 20)    # venues within 20 miles of Omaha
Venue.near([40.71, 100.23], 20)    # venues within 20 miles of a point
Venue.geocoded                     # venues with coordinates
Venue.not_geocoded                 # venues without coordinates

Some utility methods are also available:

# distance (in miles) between Eiffel Tower and Empire State Building
Geocoder.distance_between( 48.858205,2.294359,  40.748433,-73.985655 )

# look up coordinates of some location (like searching Google Maps)
Geocoder.fetch_coordinates("25 Main St, Cooperstown, NY")

# find the geographic center (aka center of gravity) of objects or points
Geocoder.geographic_center([ city1, city2, city3, [40.22,-73.99], city4 ])

More On Configuration¶ ↑

You are not stuck with using the latitude and longitude database column names for storing coordinates. For example, to use lat and lon:

geocoded_by :address, :latitude  => :lat, :longitude => :lon

The string to use for geocoding can be anything you’d use to search Google Maps. For example, any of the following are acceptable:

714 Green St, Big Town, MO
Eiffel Tower, Paris, FR
Paris, TX, US

If your model has address, city, state, and country attributes you might do something like this:

geocoded_by :location

def location
  [address, city, state, country].compact.join(', ')
end

Please see the code for more methods and detailed information about arguments (eg, working with kilometers).

SQLite¶ ↑

SQLite’s lack of trigonometric functions means Geocoder’s default implementation of the near method (named scope) does not work. When using SQLite, Geocoder will automatically use a less accurate algorithm for finding objects near a given point. Results of this algorithm should not be trusted too much as it will return objects that are outside the given radius.

Discussion¶ ↑

There are few options for finding objects near a given point in SQLite without installing extensions:

1. Use a square instead of a circle for finding nearby points. For example, if you want to find points near 40.71, 100.23, search for objects with latitude between 39.71 and 41.71 and longitude between 99.23 and 101.23. One degree of latitude or longitude is at most 69 miles so divide your radius (in miles) by 69.0 to get the amount to add and subtract from your center coordinates to get the upper and lower bounds. The results will not be very accurate (you’ll get points outside the desired radius), but you will get all the points within the required radius.

2. Load all objects into memory and compute distances between them using the Geocoder.distance_between method. This will produce accurate results but will be very slow (and use a lot of memory) if you have a lot of objects in your database.

3. If you have a large number of objects (so you can’t use approach #2) and you need accurate results (better than approach #1 will give), you can use a combination of the two. Get all the objects within a square around your center point, and then eliminate the ones that are too far away using Geocoder.distance_between.

Because Geocoder needs to provide this functionality as a named scope, we must go with option #1, but feel free to implement #2 or #3 if you need more accuracy.

To-do List¶ ↑

• prevent NameError when GOOGLE_MAPS_API_KEY is missing: show nice msg

• install.rb should do some setup when installed as a plugin

  • create initializer with GOOGLE_MAPS_API_KEY?

Copyright © 2009-10 Alex Reisner, released under the MIT license